Staff maintaining the Malahide estuary viaduct did not know its piers were not sunk into the ground, but were sitting on a man-made causeway.
Maintenance work was only carried out on the bases of the pier and not on the causeway itself.
Malahide Sea Scouts warned Iarnród Éireann a week before the viaduct collapsed that they had seen a rock at the base of one of the piers being washed away.
The engineer who received the sea scouts’ complaint thought they were referring to cracks and pointing on the pier above the water.
Climatic, oceanographic and hydrological changes contributed to the erosion of the causeway and the collapse of the viaduct.
The report recommended a handover process should be put in place to ensure knowledge is not lost when staff move or retire.
The report also noted the process for dealing with observations from the public should be documented and unified across the organisation.
It recommended a number of improvement works on the viaduct and on monitoring systems for all such bridges, which Iarnród Éireann has said it has completed or is in the process of completing.